kvm: x86: IA32_ARCH_CAPABILITIES is always supported

commit 1eaafe91a0df4157521b6417b3dd8430bf5f52f0 upstream.

If there is a possibility that a VM may migrate to a Skylake host,
then the hypervisor should report IA32_ARCH_CAPABILITIES.RSBA[bit 2]
as being set (future work, of course). This implies that
CPUID.(EAX=7,ECX=0):EDX.ARCH_CAPABILITIES[bit 29] should be
set. Therefore, kvm should report this CPUID bit as being supported
whether or not the host supports it.  Userspace is still free to clear
the bit if it chooses.

For more information on RSBA, see Intel's white paper, "Retpoline: A
Branch Target Injection Mitigation" (Document Number 337131-001),
currently available at https://bugzilla.kernel.org/show_bug.cgi?id=199511.

Since the IA32_ARCH_CAPABILITIES MSR is emulated in kvm, there is no
dependency on hardware support for this feature.

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Fixes: 28c1c9fabf48 ("KVM/VMX: Emulate MSR_IA32_ARCH_CAPABILITIES")
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 95cd58cdee9957583a13f0b029ce1ce151f0b835..6531ffcb174bd24d26496b5936411655fbedb083 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -395,6 +395,11 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
                        entry->ebx |= F(TSC_ADJUST);
                        entry->edx &= kvm_cpuid_7_0_edx_x86_features;
                        cpuid_mask(&entry->edx, 10);
+                       /*
+                        * We emulate ARCH_CAPABILITIES in software even
+                        * if the host doesn't support it.
+                        */
+                       entry->edx |= F(ARCH_CAPABILITIES);
                } else {
                        entry->ebx = 0;
                        entry->edx = 0;